The present invention relates generally to a power source for a welding machine and a method of controlling such a source, and more particularly to a welding machine power source having a high frequency zero-voltage switching converter. The disclosed power source may be used for high power welding applications, such as gas tungsten arc welding, gas metal arc welding, microjoining spot welding, seam welding, high frequency tube welding, and plasma cutting.
In the past, welding machine power sources were either of the rotating machine variety, or static converter based devices. The majority of the earlier welding machines used various static converter configurations, in conjunction with a transformer, for providing the desired voltage and current levels at the welding electrodes.
FIGS. 12 and 13 illustrate two typical earlier welding machine power sources. FIG. 12 is a schematic block diagram of a static converter based source having thyristor phase control. FIG. 13 is a schematic block diagram of a traditional inverter based welding machine power source. The controllers for generating gate signals, as well as the operator inputs, have been omitted for clarity from FIGS. 12 and 13.
Referring to FIG. 12, this traditional converter based source receives power from a three phase alternating current (AC) input. A low frequency transformer (i.e. at line frequency such as 60 Hz in the United States) is used to lower the incoming voltage level. A thyristor bridge receives this lower voltage power, and through phase-angle control, regulates the direct current (DC) output voltage or current. A choke inductor is also required at the output. These welding power sources were mainly controlled by thyristor conduction angle, as described in Electric Arc Welding Power Sources, National Electrical Manufacturers Association, Publication No. EW1, 1989. Unfortunately, these traditional static converter sources are large and heavy, have low efficiencies, slow output control responses, and high audible noise levels.
More recently, inverter-based power sources gradually replaced the traditional thyristor-controlled power sources, as discussed by H.B. Cary, in Modern Welding Technology (Prentice Hall, N.J., 1989). The inverter-based sources were physically smaller and lighter in weight than the static converter configurations. Referring to FIG. 13, a typical switch mode inverter-based source has a bridge for rectifying a three phase AC input power into a DC power. A storage capacitor C.sub.S smooths out the DC bus voltage between the rectifier bridge and an inverter bridge, which converts the DC bus voltage into a square wave AC voltage.
Typically, the inverter bridge is controlled using a pulse-width-modulation (PWM) scheme to regulate the output voltage and current. The inverter bridge output is supplied to a high frequency transformer. The ends of the transformer secondary winding are coupled through a pair of output diodes and a choke inductor to the positive electrode. A center tap of the secondary winding is coupled to the ground electrode of the welding machine.
These ordinary full-bridge inverters have two switch-pairs which turn on and off alternately to generate a square wave output. Each switch-pair contains two switching devices which turn on and off simultaneously. These simultaneous switching schemes generate high switching losses and high device switching stresses. Moreover, these earlier sources generate troublesome electromagnetic interference (EMI).
The basic idea of operating a full-bridge converter at zero-voltage crossings is described in U.S. Pat. No. 4,864,479 to Stiegerwald et al. However, Stiegerwald et al. use a DC-to-DC power supply which requires an output filter capacitor for voltage regulation. Various design considerations and a mathematical model for the Stiegerwald et al. circuit are presented in two papers. The first paper is by F.S. Tsai, and entitled "Small-Signal and Transient Analysis of a Zero-Voltage-Switched, Phase-Controlled PWM Converter Using Averaged Switch Model," Conference Record of IEEE IAS Annual Meeting, Detroit Mich. Oct. 1991, pp. 1010-1016. The second paper, by Q. Chen et al., is entitled "Optimization and Design Issues of Low Output Voltage, Off-Line, Zero-Voltage Switched PWM Converter," Proceedings of 1992 APEC Conference, Boston, Mass., Feb. 1992, pp. 73-80.
Thus, a need exists for an improved welding machine power source and an improved method of controlling such a power source, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.